1. Field of the Invention
The present invention relates to an optical multiplexer/demultiplexer that uses a waveguide to multiplex a light wave of a plurality of wavelengths into a light wave of a single wavelength or demultiplex a light wave of a single wavelength into a light wave of a plurality of wavelengths.
2. Description of the Related Art
In recent years, formats have been proposed which simultaneously transmit telephone signals at a wavelength of 1.3 [.mu.m] together with CATV and various information signals at a wavelength of 1.55 [.mu.m] using one optical fiber.
Research and development have been proceeding directed towards the realization of this technology. An optical multiplexer/demultiplexer multiplexes and demultiplexes optical signals in response to their wavelengths. For example, when carrying out the above-mentioned optical transmission and reception, an optical multiplexer/demultiplexer is used in order to multiplex an optical signal with a wavelength band of 1.3 [.mu.m] and an optical signal with a wavelength band of 1.55 [.mu.m] or demultiplex an optical signal that was multiplexed.
For the composition of an optical multiplexer/demultiplexer, devices which combine elements, fiber types, and waveguide types are known.
An optical multiplexer/demultiplexer that combines elements comprises elements which spatially divide light waves according to their wavelengths by prisms, diffraction gratings or interference membrane filters and elements which convert the optical paths of mirrors, lenses and other similar devices.
This optical multiplexer/demultiplexer has problems such as difficulties in size reduction due to a complex composition, high assembly and adjustment costs, and low tolerance to heat and shocks.
A fiber type optical multiplexer/demultiplexer combines an optical brancher and a directional coupler which are produce[dB]y fusing two or more optical fibers. Further, in a fiber type optical multiplexer/demultiplexer, a grating mirror is formed within the optical fiber.
This type of optical multiplexer/demultiplexer has problems such as in size reduction as well as difficulties in setting the target wavelength.
A waveguide type optical multiplexer/demultiplexer is comprised by directional couplers and Mach-Zehnder interferometers formed within the substrate or on the substrate by means of waveguides. This optical multiplexer/demultiplexer has great freedom in wavelength selectivity in the design. Further, this optical multiplexer/demultiplexer has advantageous of small size and uniform characteristics leading to low-cost manufacture due to the fact that is uses micro-machining technology like as ICs production.
As a conventional example of this type of optical multiplexer/demultiplexer, for example, directional couplers and Mach-Zehnder interferometers have been published in the Electron Information Transmission Journal C-I, Vol. J73-C-I, No. 5, p354-359, 1990.
FIG. 9 is a top view showing a waveguide type optical multiplexer/demultiplexer 50 comprised by the above-mentioned directional coupler published in the Electron Information Transmission Journal.
A first waveguide 53 and a second waveguide 54 are formed on a substrate 51. This first waveguide 53 and second waveguide 54 are in close proximity to each other at the mid-point of the substrate 51. This portion that is in close proximity constitutes a directional coupler 52. The coupling length (length of the portion that is in close proximity of each waveguide 53, 54) of this directional coupler 52 becomes equal to an odd number multiple of the minimum perfect coupling length (the minimum coupling length when light output from a waveguide is admitted and is completely transferred to the other waveguide) for a certain wavelength .lambda..sub.1. The coupling length is further set such that it becomes equal to an odd number multiple of the minimum coupling length for a separate wavelength .lambda..sub.2. Because of this, an optical signal of wavelength .lambda..sub.1 admitted into the first waveguide 53 will output from the second waveguide 54. Further, an optical signal of wavelength .lambda..sub.2 admitted into the second waveguide 54 will also output from the second waveguide 54. By this function, each optical signal of wavelength .lambda..sub.1 and wavelength .lambda..sub.2 are multiplexed.
Even further, an optical signal of wavelength .lambda..sub.1 admitted into the first waveguide 53 will output from the second waveguide 54. Further, an optical signal of wavelength .lambda..sub.2 admitted into the first waveguide 53 will output from the first waveguide 53. By this function, each optical signal of wavelength .lambda..sub.1 and wavelength .lambda..sub.2 which were multiplexed are demultiplexed.
FIG. 10 is a top view showing a Mach-Zehnder interferometer type optical multiplexer/demultiplexer 60 as published in the above-mentioned Electron Information Transmission Journal.
A first waveguide 63 and a second waveguide 64 are formed on a substrate 61. This first waveguide 63 and second waveguide 64 come into close proximity to each other at two locations within their paths. These portions which are in close proximity form a first 3 [dB] coupler 65 and a second 3 [dB] coupler 66. This first waveguide 63 and second waveguide 64 are set such that a fixed difference in the optical path occur at the portion sandwiched between the first 3 [dB] coupler 65 and the second 3 [dB] coupler 66. This fixed difference in the optical path becomes an integral number multiple of .lambda..sub.1 for a certain wavelength .lambda..sub.1. This fixed difference in the optical path is further set such that it becomes a half-integral number multiple of .lambda..sub.2 for a separate wavelength .lambda..sub.2.
Because of this, an optical signal of wavelength .lambda..sub.1 admitted into the first waveguide 63 will output from the second waveguide 64. Further, an optical signal of wavelength .lambda..sub.2 admitted into the second waveguide 64 will also output from the second waveguide 64. By this function, each optical signal of wavelength .lambda..sub.1 and wavelength .lambda..sub.2 are multiplexed.
Even further, an optical signal of wavelength .lambda..sub.1 admitted into the first waveguide 13 will output from the second waveguide 14. Further, an optical signal of wavelength .lambda..sub.2 admitted into the first waveguide 13 will output from the first waveguide 13. By this function, each optical signal of wavelength .lambda..sub.1 and wavelength .lambda..sub.2 which were multiplexed are demultiplexed.
The following problems are present in the conventional directional coupler type optical multiplexer/demultiplexer 50 shown in FIG. 9.
The first problem is that because the properties of the directional coupler 52 are extremely sensitive to variations in the wavelength, the wavelength band of the optical signal that can pass through the coupler narrows making it impossible to include many wavelengths within the wavelength passing band.
The second problem is that because the properties of the directional coupler 52 are extremely sensitive to element parameters of the coupler portions, the allowable production errors of the coupler portions grow smaller in number making it difficult to stably produce elements with low crosstalk between the wavelengths.
Moreover, the following problems are present in the Mach-Zehnder interferometer type optical multiplexer/demultiplexer shown in FIG. 10.
The first problem is that because there is a wavelength dependency on the branching ratio of the directional coupler used as the 3 [dB] couplers 65, 66, it is difficult to design a multiplexer/demultiplexer that corresponds to a wide wavelength band.
The second problem is that because it is necessary to separate the two 3 [dB] couplers 65, 66 by a certain distance in order than coupling does not occur, the length of the waveguide almost doubles compared to a directional coupler and the size of the elements increases compared to a directional coupler.